The present invention relates to a calibration method for measuring an optical transmission factor of exhaust gas or contaminated atmosphere. The present invention has an application, for instance, in a smoke indicator, a dust concentration indicator, or a gas combustion control, etc.
The present invention provides some considerable improvement on the optical transmission measurement apparatus being disclosed in U.S. Pat. No. 4,726,684, which was invented by the same inventor and has been assigned to the assignee of the present invention.
In the '684 patent, the inventor of the present invention has disclosed an original measurement device and method that measures the optical transmission factor with a pair of intercommunicating laser beams.
In this patent, it is an essential process to detect how much of the optical energy of each of the laser beams is reduced as the beams pass through exhaust gas, combustion gas, or other object. And it is another essential process to provide an indication of how much of each of the laser beams goes through the object when the object does not disturb the laser beams.
Practically in the apparatus, a pair of photo-detectors detect the optical energies of the laser beams which have passed through the object. By the detection, the optical transmission factor of the object would be determined as a relative percentage between 0 and 100%. So, in order to determine the relative percentage, the levels of the output signals from the photo-detectors should be predetermined, according to the reduction of the energy of the laser beams under two different conditions. One of the two conditions is where the object does not disturb the laser beams. The other condition is where the laser beams do not reach the photo-detectors because the laser beams are completely shut out by the object.
In order to determine the levels of the signals, one simple technique is to examine the output signals when the laser beams approach the photo-detectors directly without any reduction, and when the laser beams do not reach the photo-detectors. For providing these conditions, one way is for the object to be clear fresh air, a path through which can be blocked with an opaque light-disturbing device.
However, it is a technical problem that the optical energies of the laser beams are reduced not only through the object, but also in interactions with other optical devices. One of those devices is the polarized beam splitter. Although the optical reduction factor of the polarized beam splitter itself is determinate, the optical reduction factor of dirt, blurs, or scratches on the surface of the polarized beam splitter is indeterminate, or unclear. The condition of the beam splitter will change during the measurement because the air to be measured will provide some dust or soil on the beam splitter surface, or cause the beam splitter itself to deteriorate over time.
Those optical reductions in the polarized beam splitter will disturb the determination of the exact relative optical transmission by biasing the standard condition of zero reduction, or that of complete reduction, of the optical energies of the laser beams.
In addition, it is another problem that providing clean air during the measurement is very troublesome. In order to provide clean air in place of the object, it is necessary to remove the exhaust gas to be measured. Upon removing the air to be measured, it becomes impossible to measure the optical transmission factor. Removing the exhaust gas completely enables calibration to be performed. After the calibration, the clean air must be replaced with the exhaust gas to be measured again.
Thus, in the air replacing process described above, great difficulties cannot be avoided.
In order to avoid those difficulties concerned with the calibrations, the present invention will provide an advanced calibration device and a simple calibration process for the apparatus of the '684 patent.
One purpose of the present invention is to provide an advanced optical transmission apparatus and method that enables the calibration to be performed without replacing exhaust gas with clean air.
Another purpose of the present invention is to provide an advanced optical transmission apparatus and method that enables a speedy calibration that takes only a short time during the measurement.
And still another purpose of the present invention is to provide an advanced optical transmission apparatus and method that enables an automatic calibration to be performed without manual operation, that is of great help for maintaining accuracy during long term continuous measurement.